Nanostructured materials for battery applications

ABSTRACT

The present invention relates to nanostructured materials (including nanowires) for use in batteries. Exemplary materials include carbon-comprising, Si-based nanostructures, nanostructured materials disposed on carbon-based substrates, and nanostructures comprising nanoscale scaffolds. The present invention also provides methods of preparing battery electrodes, and batteries, using the nanostructured materials.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/599,795, filed on Oct. 11, 2019, currently pending, which is acontinuation of U.S. patent application Ser. No. 12/783,243, filed onMay 19, 2010, now U.S. Pat. No. 10,490,817, which claims the benefit ofU.S. Provisional Application No. 61/179,663, entitled “Nanowire EnabledBattery Technology”, filed May 19, 2009, 61/221,392 entitled“Nanostructured Materials For Battery Applications”, filed Jun. 29,2009, and 61/255,732 entitled “Nanostructured Materials For BatteryApplications”, filed Oct. 28, 2009, which are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to nanostructured materials (includingnanowires) for use in batteries. Exemplary materials includecarbon-comprising, Si-based nanostructures, nanostructured materialsdisposed on carbon-based substrates, and nanostructures comprisingnanoscale scaffolds. The present invention also provides methods ofpreparing battery electrodes, and batteries, using the nanostructuredmaterials.

Background of the Invention

Traditional batteries, including lithium (Li) ion batteries, comprise ananode, a separator material with an electrolyte medium, and a cathode.The anode electrode of most commercially available Li ion batteries is acopper foil coated with a mixture of graphite powder and a polymer blendsuch as polyvinylidene difluoride (PVDF). The cathode generallycomprises a mixture of lithium transition metal oxides, PVDF and carbonblack coated onto an aluminum foil. While graphite and LiCoO₂ arenormally used, and have high coulombic efficiencies, there is a need forimproved anode materials with superior storage capacity and thermalstability.

One material that has attracted a great deal of attention is silicon(Si), due to its high capacity. However, repeated charging/dischargingcycles have been found to cause a volume change in the Si, which canultimately cause the material to disintegrate and severely limit anypotential battery life. Thus, there is a need for improved electrodematerials for use in batteries, and specifically, Si-based materials.

SUMMARY OF THE INVENTION

In one embodiment the present invention provides additives for use in abattery slurry. Suitably, such additives comprise one or morecarbon-comprising, Si-based nanostructures. Exemplary Si-basednanostructures include Si-based nanowires and Si-based nanoparticles.Suitably, the nanowires have a core-shell structure, and in exemplaryembodiments, the core comprises Si, and the shell comprises C. TheSi-based nanowires suitably have a diameter of about 20 nm to about 200nm, and a length of about 0.1 pm to about 50 μm.

In embodiments, the additives comprise about 1 weight % to about 80weight % (more suitably about 10 weight %) of the slurry. In exemplaryembodiments, a conductive polymer, such as polyvinylidene difluoride, isdisposed on the Si-based nanostructures. The present invention alsoprovides battery slurries comprising one or more carbon-comprising,Si-based nanostructures. Exemplary characteristics of the Si-basednanostructures, including Si-based nanowires, are described herein.Suitably, the battery slurries comprise about 1 weight % to about 80weight % (suitably about 10 weight %) of the carbon-comprising, Si-basednanostructures. In exemplary embodiments, the battery slurries furthercomprise a carbon-based material, such as carbon or graphite.

In another embodiment, the additives of the present invention compriseone or more nanostructures disposed on a carbon-based substrate.Exemplary nanostructures include nanowires or nanoparticles, such asnanowires having a core-shell structure. Suitably, the nanowirescomprise a crystalline core (e.g., Si) and non-oxide, amorphous shell(e.g., Si or C). In exemplary embodiments, the nanowires ornanoparticles comprise Si. Suitably, the nanowires have a diameter ofabout 20 nm to about 200 nm, and a length of about 0.1 μm to about 50μm.

Exemplary carbon-based substrates include a carbon-based powder, carbonblack, graphite, graphene, graphene powder and graphite foil. Suitably,the carbon-based powder includes particles of about 5 microns to about50 microns, e.g., about 20 microns. Suitably, the additives of thepresent invention comprise about 1 weight % to about 80 weight % (e.g.,about 10 weight %) of the slurry. The additives can further comprise aconductive polymer, such as polyvinylidene difluoride, disposed on thenanostructures.

In a further embodiment, the additives of the present invention compriseone or more nanostructures comprising a nanoscale scaffold, a Si-basedlayer disposed on the nanoscale scaffold and a carbon-based layerdisposed on the Si-based layer. Exemplary nanoscale scaffolds includenanowires, nanofibers, and nanotubes. Suitably, the nanowires have adiameter of about 20 nm to about 200 nm, and a length of about 0.1 μm toabout 50 μm.

The present invention also provides a battery slurry and/or batteryelectrodes (e.g., anodes) comprising one or more of thecarbon-comprising Si-based nanostructures, the nanostructures disposedon a carbon-based substrate, and/or the nanostructures comprising ananoscale scaffold, as described herein. Exemplary nanostructures,including compositions and characteristics of the nanostructures aredescribed throughout. In exemplary embodiments, the nanostructures, suchas nanowires, comprise Li inserted in the nanowires. In exemplaryembodiments, the electrodes comprise about 1 weight % to about 80 weight% (e.g., about 10 weight %) of the nanostructures. Suitably, thenanostructures are embedded in a Li foil.

The present invention also provides batteries having an anode comprisingone or more of the nanostructures of the present invention. Suitably,the batteries are Li ion batteries. The batteries of the presentinvention also suitably further comprise a cathode, and an electrolyteseparator positioned between the anode and the cathode. Exemplarycathode materials include, but are not limited to, LiCoO₂, LiFePO₄,LiMnO₂, LiMnO₄, LiNiCoA1O/LiNiCoMnO⁺LiMn₂O₄, LiCoFePO₄ and LiNiO₂.Suitably, the batteries further comprise a housing encasing the anode,the electrolytic separator and the cathode.

The present invention also provides methods of preparing a batteryelectrode. Suitably, the methods comprise providing one or more of thenanostructures of the present invention. The nanostructures are mixedwith a conductive polymer and a carbon-based material to form a slurry.The slurry is formed into the battery electrode.

The present invention also provides methods of preparing a battery.Suitably, the methods comprise providing one or more nanostructures. Thenanostructures are mixed with a conductive polymer and a carbon-basedmaterial to form a slurry. The slurry is formed into a battery anode,and a separator is disposed between the anode and a cathode.

The present invention also provides methods of preparing a carbon-coatednanostructure. Suitably, the methods comprise providing a nanoscalescaffold. A carbon-comprising polymer is disposed on the nanoscalescaffold. The carbon-comprising polymer is heated to form a carboncoating on the nanoscale scaffold.

The present invention further provides methods for preparing an additivefor use in a battery slurry. Suitably, the methods comprise providing acarbon-based powder. A Si-based nanostructure is disposed on thecarbon-based powder.

Further embodiments, features, and advantages of the invention, as wellas the structure and operation of the various embodiments of theinvention are described in detail below with reference to accompanyingdrawings.

BRIEF DESCRIPTION OF THE FIGURES

The invention is described with reference to the accompanying drawings.In the drawings, like reference numbers indicate identical orfunctionally similar elements. The drawing in which an element firstappears is indicated by the left-most digit in the correspondingreference number.

FIG. 1A shows a carbon-comprising, Si-based nanostructure in accordancewith an embodiment of the present invention.

FIG. 1B shows nanostructures disposed on carbon-based substrates inaccordance with an embodiment of the present invention.

FIG. 1C shows a battery electrode of an embodiment of the presentinvention.

FIG. 1D shows a carbon-coated nanostructure of an embodiment of thepresent invention.

FIG. 1E shows nanostructures disposed on a carbon-based powder inaccordance with an embodiment of the present invention.

FIG. 1F shows a battery electrode of an embodiment of the presentinvention.

FIG. 2 shows a battery in accordance with an embodiment of the presentinvention.

FIGS. 3A-3B show flowcharts of methods of preparing a battery electrodein accordance with embodiments of the present invention.

FIGS. 4A-4B show flowcharts of methods of preparing a battery inaccordance with embodiments of the present invention.

FIGS. 5A-5B show scanning electron microscopy (SEM) micrographs ofnanowires grown with a high degree of straightness and verticality (A)and with a random, interweaving, intertwining and overlappingorientation (B).

FIG. 6 shows an SEM micrograph of silicon nanowires grown on carbonblack.

FIGS. 7A and 7B show SEM micrographs of silicon nanowires grown ongraphite foil at low (A) and high (B) magnification.

FIGS. 8A and 8B show SEM micrographs of loose graphene microsheetpowders (A) and silicon nanowires grown on the graphene powder (B).

FIG. 9 shows a transmission electron microscopy (TEM) micrograph ofsilicon nanowires with a crystalline core and amorphous shell.

FIG. 10 shows charge capacity (solid markers) and cycle efficiencies(open markers) for silicon nanowires with two different diameters grownon steel substrates.

FIG. 11 shows current versus potential curves for silicon nanowires withdifferent diameters grown on stainless steel substrate.

FIG. 12 shows current versus potential curves for a silicon thin filmand silicon thin film plus silicon nanowires, both grown on stainlesssteel substrates.

FIG. 13 shows capacity as a function of Charge/Discharge Cycle comparingcompositions of the present invention to control compositions.

FIG. 14 shows an SEM micrograph of silicon nanowires after 60 chargecycles.

FIG. 15 shows a comparison of the fast charge cycling behavior of a cellcomprising a Li-silicon nanowire anode/LiCO₂ cathode compared to a Lianode/LiCO₂ cathode control cell.

FIGS. 16A-16C show scanning transmission electron microscope (STEM)Energy Dispersive X-ray (EDX) micrographs revealing the uniform andhomogenous distribution of carbon (16B) and lead (Pb) as marker forNAFION® (16C) on the silicon nanowire network (16A).

FIG. 17 shows an exemplary process for producing nanowires in accordancewith and embodiment of the present invention.

FIG. 18 shows an exemplary process/equipment design for introducing theadditives of the present invention into existing slurry preparations.

FIG. 19 shows a flowchart of a method of preparing a carbon-coatednanostructure in accordance with an embodiment of the present invention.

FIG. 20 shows a flowchart of a method of preparing an additive for usein a battery slurry in accordance with an embodiment of the presentinvention.

FIGS. 21A-21B show micrographs of a nanostructure comprising a carboncoating in accordance with an embodiment of the present invention.

FIG. 22 shows a micrograph of a nanostructure comprising a carboncoating in accordance with another embodiment of the present invention.

FIGS. 23A-23D show micrographs of nanostructures of embodiments of thepresent invention, illustrating morphology changes after severalcharge/discharge cycles.

FIGS. 24A and 24B show SEM micrographs of silicon nanowires at low (A)and high (B) magnification.

FIGS. 25A and 25B show TEM micrographs of silicon nanowires with acrystalline core and a combination of an amorphous Si and poly-Si shell.

FIG. 26 shows Fourier Transform Infrared Spectroscopy (FTIR)measurements, illustrating differences between SiNWs and Si powders.

FIG. 27 shows a graph of capacity as a function of cycle number for afirst anode comprising 10% Si nanowires, 10% PVDF, and 80% graphitecarbon, and a second anode comprising only graphite carbon and PVDF.

DETAILED DESCRIPTION OF THE INVENTION

It should be appreciated that the particular implementations shown anddescribed herein are examples of the invention and are not intended tootherwise limit the scope of the present invention in any way. Indeed,for the sake of brevity, conventional electronics, manufacturing,semiconductor devices, and nanowire (NW), nanorod, nanotube, andnanoribbon technologies and other functional aspects of the systems (andcomponents of the individual operating components of the systems) maynot be described in detail herein. Furthermore, for purposes of brevity,the invention is frequently described herein as pertaining to nanowires,though other similar structures are also encompassed herein.

It should be appreciated that although nanowires are frequently referredto, the techniques described herein are also applicable to othernanostructures, such as nanorods, nanoparticles, nanopowder, nanotubes,nanotetrapods, nanoribbons, nanosheets and/or combinations thereof.

As used herein, an “aspect ratio” is the length of a first axis of ananostructure divided by the average of the lengths of the second andthird axes of the nanostructure, where the second and third axes are thetwo axes whose lengths are most nearly equal to each other. For example,the aspect ratio for a perfect rod would be the length of its long axisdivided by the diameter of a cross-section perpendicular to (normal to)the long axis.

The term “heterostructure” when used with reference to nanostructuresrefers to nanostructures characterized by at least two different and/ordistinguishable material types. Typically, one region of thenanostructure comprises a first material type, while a second region ofthe nanostructure comprises a second material type. In anotherembodiment, the nanostructure comprises a core of a first material andat least one shell of a second (or third etc.) material, where thedifferent material types are distributed radially about the long axis ofa nanowire, a long axis of an arm of a branched nanocrystal, or thecenter of a nanocrystal, for example. A shell need not completely coverthe adjacent materials to be considered a shell or for the nanostructureto be considered a heterostructure. For example, a nanocrystalcharacterized by a core of one material covered with small islands of asecond material is a hetero structure. In other embodiments, thedifferent material types are distributed at different locations withinthe nanostructure. For example, material types can be distributed alongthe major (long) axis of a nanowire or along a long axis or arm of abranched nanocrystal. Different regions within a hetero structure cancomprise entirely different materials, or the different regions cancomprise a base material.

As used herein, a “nanostructure” is a structure having at least oneregion or characteristic dimension with a dimension of less than about500 nm, e.g., less than about 200 nm, less than about 100 nm, less thanabout 50 nm, or even less than about 20 nm. Typically, the region orcharacteristic dimension will be along the smallest axis of thestructure. Examples of such structures include nanowires, nanopowder,nanorods, nanofilms, nanotubes, branched nanocrystals, nanotetrapods,tripods, bipods, nanocrystals, nanodots, quantum dots, nanoparticles,branched tetrapods (e.g., inorganic dendrimers), and the like.Nanostructures can be substantially homogeneous in material properties,or in other embodiments can be heterogeneous (e.g., heterostructures).Nanostructures can be, for example, substantially crystalline,substantially monocrystalline, polycrystalline, amorphous, orcombinations thereof In one aspect, one of the three dimensions of thenanostructure has a dimension of less than about 500 nm, for example,less than about 200 nm, less than about 100 nm, less than about 50 nm,or even less than about 20 nm.

As used herein, the term “nanowire” generally refers to any elongatedconductive or semiconductive material (or other material describedherein) that includes at least one cross sectional dimension that isless than 500 nm, and suitably, less than 200 nm, or less than 100 nm,and has an aspect ratio (length:width) of greater than 10, preferablygreater than 50, and more preferably, greater than 100, for example, upto about 1000, or more.

As used herein, a “nanoparticle” refers to a particle, crystal, sphere,or other shaped structure having at least one region or characteristicdimension with a dimension of less than about 500 nm, suitably less thanabout 200 nm, less than about 100 nm, less than about 50 nm, less thanabout 20 nm, or less than about 10 nm. Suitably, all of the dimensionsof the nanoparticles utilized in the present invention are less thanabout 50 nm, and suitably have a size of about 1 nm to about 30 nm, orabout 1 nm to about 20 nm, about 1 nm to about 10 nm, about 1 nm toabout 9 nm, about 1 nm to about 8 nm, about 1 nm to about 7 nm, about 1nm to about 6 nm, about 1 nm to about 5 nm, about 1 nm to about 4 nm,about 1 nm to about 3 nm, or about 1 nm to about 2 nm, for example,about 1 nm, about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm,about 7 nm, about 8 nm, about 9 nm, or about 10 nm.

The nanowires of this invention can be substantially homogeneous inmaterial properties, or in other embodiments can be heterogeneous (e.g.nanowire heterostructures). The nanowires can be fabricated fromessentially any convenient material or materials, and can be, e.g.,substantially crystalline, substantially monocrystalline,polycrystalline, amorphous, or combinations thereof. Nanowires can havea variable diameter or can have a substantially uniform diameter, thatis, a diameter that shows a variance less than about 20% (e.g., lessthan about 10%, less than about 5%, or less than about 1%) over theregion of greatest variability and over a linear dimension of at least 5nm (e.g., at least 10 nm, at least 20 nm, or at least 50 nm). Typicallythe diameter is evaluated away from the ends of the nanowire (e.g., overthe central 20%, 40%, 50%, or 80% of the nanowire). A nanowire can bestraight or can be e.g., curved or bent, over the entire length of itslong axis or a portion thereof. In other embodiments, a nanowire or aportion thereof can exhibit two- or three-dimensional quantumconfinement.

Examples of such nanowires include semiconductor nanowires as describedin Published International Patent Application Nos. WO 02/017362, WO02/048701, and WO 01/003208, carbon nanotubes, and other elongatedconductive or semiconductive structures of like dimensions, which areincorporated herein by reference.

As used herein, the term “nanorod” generally refers to any elongatedconductive or semiconductive material (or other material describedherein) similar to a nanowire, but having an aspect ratio (length:width)less than that of a nanowire. Note that two or more nanorods can becoupled together along their longitudinal axis. Alternatively, two ormore nanorods can be substantially aligned along their longitudinalaxis, but not coupled together, such that a small gap exists between theends of the two or more nanorods. In this case, electrons can flow fromone nanorod to another by hopping from one nanorod to another totraverse the small gap. The two or more nanorods can be substantiallyaligned, such that they form a path by which electrons can travel.

A wide range of types of materials for nanostructures, includingnanowires, nanocrystals, nanoparticles, nanopowders, nanorods, nanotubesand nanoribbons can be used, including semiconductor material selectedfrom, e.g., Si, Ge, Sn, Se, Te, B, C (including diamond), P, BC,BP(BP₆), BSi, SiC, SiGe, SiSn, GeSn, WC, SiO₂, TiO₂, BN, BAs, A1N, A1P,A1As, A1Sb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, ZnO, ZnS, ZnSe, ZnTe,CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe, MgS, MgSe, GeS, GeSe,GeTe, SnS, SnSe, SnTe, PbO, PbS, Pb Se, PbTe, CuF, CuCl, CuBr, CuI, AgF,AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂, CuGeP₃,CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄, Al₂O₃,(Al, Ga, In)₂ (S, Se, Te)₃, Al₂CO, and an appropriate combination of twoor more such semiconductors.

Nanowires of the present invention may also comprise organic polymers,ceramics, inorganic semiconductors such as carbides and nitrides, andoxides (such as TiO₂ or ZnO), carbon nanotubes, biologically derivedcompounds, e.g., fibrillar proteins, etc. or the like. For example, incertain embodiments, inorganic nanowires are employed, such assemiconductor nanowires. Semiconductor nanowires can be comprised of anumber of Group IV, Group III-V or Group II-VI semiconductors or theiroxides. In one embodiment, the nanowires may include metallicconducting, semiconducting, carbide, nitride, or oxide materials such asRuO₂, SiC, GaN, TiO₂, SnO₂, WC_(x), MoC_(x), ZrC, WN_(x), MoN_(x) etc.As used throughout, the subscript “x,” when used in chemical formulae,refers to a whole, positive integer (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9,10, etc). It is suitable that the nanowires be made from a material thatis resistant to degradation in a weak acid. Nanowires according to thisinvention can include, or can expressly exclude, carbon nanotubes, and,in certain embodiments, exclude “whiskers” or “nanowhiskers”,particularly whiskers having a diameter greater than 100 nm, or greaterthan about 200 nm.

In other aspects, the semiconductor may comprise a dopant from a groupconsisting of: a p-type dopant from Group III of the periodic table; ann-type dopant from Group V of the periodic table; a p-type dopantselected from a group consisting of: B, Al and In; an n-type dopantselected from a group consisting of: P, As and Sb; a p-type dopant fromGroup II of the periodic table; a p-type dopant selected from a groupconsisting of: Mg, Zn, Cd and Hg; a p-type dopant from Group IV of theperiodic table; a p-type dopant selected from a group consisting of: Cand Si.; or an n-type dopant selected from a group consisting of: Si,Ge, Sn, S, Se and Te. Other now known or later developed dopantmaterials can be employed.

Additionally, the nanowires can include carbon nanotubes, or nanotubesformed of conductive or semiconductive organic polymer materials, (e.g.,pentacene, and transition metal oxides).

It should be understood that the spatial descriptions (e.g., “above”,“below”, “up”, “down”, “top”, “bottom”, etc.) made herein are forpurposes of illustration only, and that devices of the present inventioncan be spatially arranged in any orientation or manner.

Nanostructures have been produced in a wide variety of different ways.For example, solution based, surfactant mediated crystal growth has beendescribed for producing spherical inorganic nanomaterials, e.g., quantumdots, as well as elongated nanomaterials, e.g., nanorods andnanotetrapods. Other methods have also been employed to producenanostructures, including vapor phase methods. For example, siliconnanocrystals have been reportedly produced by laser pyrolysis of silanegas.

Other methods employ substrate based synthesis methods including, e.g.,low temperature synthesis methods for producing, e.g., ZnO nanowires asdescribed by Greene et al. (“Low-temperature wafer scale production ofZnO nanowire arrays,” L. Greene, M. Law, J. Goldberger, F. Kim, J.Johnson, Y. Zhang, R. Saykally, P. Yang, Angew. Chem. Int. Ed. 42,3031-3034, 2003), and higher temperature vapor-liquid-solid (VLS)methods that employ catalytic gold particles, e.g., that are depositedeither as a colloid or as a thin film that forms a particle uponheating. Such VLS methods of producing nanowires are described in, forexample, Published International Patent Application No. WO 02/017362,the full disclosure of which is incorporated herein by reference in itsentirety for all purposes.

Nanostructures can be fabricated and their size can be controlled by anyof a number of convenient methods that can be adapted to differentmaterials. For example, synthesis of nanocrystals of various compositionis described in, e.g., Peng et al. (2000) “Shape Control of CdSeNanocrystals” Nature 404, 59-61; Puntes et al. (2001) “Colloidalnanocrystal shape and size control: The case of cobalt” Science 291,2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos et al. (Oct. 23, 2001)entitled “Process for forming shaped group III-V semiconductornanocrystals, and product formed using process;” U.S. Pat. No. 6,225,198to Alivisatos et al. (May 1, 2001) entitled “Process for forming shapedgroup II-VI semiconductor nanocrystals, and product formed usingprocess;” U.S. Pat. No. 5,505,928 to Alivisatos et al. (Apr. 9, 1996)entitled “Preparation of III-V semiconductor nanocrystals;” U.S. Pat.No. 5,751,018 to Alivisatos et al. (May 12, 1998) entitled“Semiconductor nanocrystals covalently bound to solid inorganic surfacesusing self-assembled monolayers;” U.S. Pat. No. 6,048,616 to Gallagheret al. (Apr. 11, 2000) entitled “Encapsulated quantum sized dopedsemiconductor particles and method of manufacturing same;” and U.S. Pat.No. 5,990,479 to Weiss et al. (Nov. 23, 1999) entitled “Organoluminescent semiconductor nanocrystal probes for biological applicationsand process for making and using such probes.” The disclosures of eachof these publications are incorporated by reference herein in theirentireties.

Growth of nanowires having various aspect ratios, including nanowireswith controlled diameters, is described in, e.g., Gudiksen et al. (2000)“Diameter-selective synthesis of semiconductor nanowires” J. Am. Chem.Soc. 122, 8801-8802; Cui et al. (2001) “Diameter-controlled synthesis ofsingle-crystal silicon nanowires” Appl. Phys. Lett. 78, 2214-2216;Gudiksen et al. (2001) “Synthetic control of the diameter and length ofsingle crystal semiconductor nanowires” J. Phys. Chem. B 105, 4062-4064;Morales et al. (1998) “A laser ablation method for the synthesis ofcrystalline semiconductor nanowires” Science 279, 208-211; Duan et al.(2000) “General synthesis of compound semiconductor nanowires” Adv.Mater. 12, 298-302; Cui et al. (2000) “Doping and electrical transportin silicon nanowires” J. Phys. Chem. B 104, 5213-5216; Peng et al.(2000) “Shape control of CdSe nanocrystals” Nature 404, 59-61; Puntes etal. (2001) “Colloidal nanocrystal shape and size control: The case ofcobalt” Science 291, 2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos etal. (Oct. 23, 2001) entitled “Process for forming shaped group III-Vsemiconductor nanocrystals, and product formed using process;” U.S. Pat.No. 6,225,198 to Alivisatos et al. (May 1, 2001) entitled “Process forforming shaped group II-VI semiconductor nanocrystals, and productformed using process”; U.S. Pat. No. 6,036,774 to Lieber et al. (Mar.14, 2000) entitled “Method of producing metal oxide nanorods”; U.S. Pat.No. 5,897,945 to Lieber et al. (Apr. 27, 1999) entitled “Metal oxidenanorods”; U.S. Pat. No. 5,997,832 to Lieber et al. (Dec. 7, 1999)“Preparation of carbide nanorods;” Urbau et al. (2002) “Synthesis ofsingle-crystalline perovskite nanowires composed of barium titanate andstrontium titanate” J. Am. Chem. Soc., 124, 1186; and Yun et al. (2002)“Ferroelectric Properties of Individual Barium Titanate NanowiresInvestigated by Scanned Probe Microscopy” Nanoletters 2, 447. Thedisclosures of each of these publications are incorporated by referenceherein in their entireties.

In certain embodiments, the nanowires of the present invention areproduced by growing or synthesizing these elongated structures onsubstrate surfaces. By way of example, published U.S. Patent ApplicationNo. US-2003-0089899-A1 (the disclosure of which is incorporated byreference herein) discloses methods of growing uniform populations ofsemiconductor nanowires from gold colloids adhered to a solid substrateusing vapor phase epitaxy/VLS. Greene et al. (“Low-temperature waferscale production of ZnO nanowire arrays”, L. Greene, M. Law, J.Goldberger, F. Kim, J. Johnson, Y. Zhang, R. Saykally, P. Yang, Angew.Chem. Int. Ed. 42, 3031-3034, 2003) discloses an alternate method ofsynthesizing nanowires using a solution based, lower temperature wiregrowth process. A variety of other methods are used to synthesize otherelongated nanomaterials, including the surfactant based syntheticmethods disclosed in U.S. Pat. Nos. 5,505,928, 6,225,198 and 6,306,736,for producing shorter nanomaterials, and the known methods for producingcarbon nanotubes, see, e.g., US-2002/0179434 to Dai et al., as well asmethods for growth of nanowires without the use of a growth substrate,see, e.g., Morales and Lieber, Science, V. 279, p. 208 (Jan. 9, 1998).As noted herein, any or all of these different materials may be employedin producing the nanowires for use in the invention. For someapplications, a wide variety of group III-V, II-VI and group IVsemiconductors may be utilized, depending upon the ultimate applicationof the substrate or article produced. In general, such semiconductornanowires have been described in, e.g., US-2003-0089899-A1, incorporatedherein above. The disclosures of each of these publications areincorporated by reference herein in their entireties.

Growth of branched nanowires (e.g., nanotetrapods, tripods, bipods, andbranched tetrapods) is described in, e.g., Jun et al. (2001) “Controlledsynthesis of multi-armed CdS nanorod architectures using monosurfactantsystem” J. Am. Chem. Soc. 123, 5150-5151; and Manna et al. (2000)“Synthesis of Soluble and Processable Rod-, Arrow-, Teardrop-, andTetrapod-Shaped CdSe Nanocrystals” J. Am. Chem. Soc. 122, 12700-12706.The disclosures of each of these publications are incorporated byreference herein in their entireties.

Synthesis of nanoparticles is described in, e.g., U.S. Pat. No.5,690,807 to Clark Jr. et al. (Nov. 25, 1997) entitled “Method forproducing semiconductor particles”; U.S. Pat. No. 6,136,156 to El-Shall,et al. (Oct. 24, 2000) entitled “Nanoparticles of silicon oxide alloys;”U.S. Pat. No. 6,413,489 to Ying et al. (Jul. 2, 2002) entitled“Synthesis of nanometer-sized particles by reverse micelle mediatedtechniques;” and Liu et al. (2001) “Sol-Gel Synthesis of Free-StandingFerroelectric Lead Zirconate Titanate Nanoparticles” J. Am. Chem. Soc.123, 4344. The disclosures of each of these publications areincorporated by reference herein in their entireties. Synthesis ofnanoparticles is also described in the above citations for growth ofnanocrystals, nanowires, and branched nanowires, where the resultingnanostructures have an aspect ratio less than about 1.5.

Synthesis of core-shell nanostructure heterostructures, namelynanocrystal and nanowire (e.g., nanorod) core-shell heterostructures,are described in, e.g., Peng et al. (1997) “Epitaxial growth of highlyluminescent CdSe/CdS core/shell nanocrystals with photostability andelectronic accessibility” J. Am. Chem. Soc. 119, 7019-7029; Dabbousi etal. (1997) “(CdSe)ZnS core-shell quantum dots: Synthesis andcharacterization of a size series of highly luminescent nanocrysallites”J. Phys. Chem. B 101, 9463-9475; Manna et al. (2002) “Epitaxial growthand photochemical annealing of graded CdS/ZnS shells on colloidal CdSenanorods” J. Am. Chem. Soc. 124, 7136-7145; and Cao et al. (2000)“Growth and properties of semiconductor core/shell nanocrystals withInAs cores” J. Am. Chem. Soc. 122, 9692-9702. Similar approaches can beapplied to growth of other core-shell nanostructures. The disclosures ofeach of these publications are incorporated by reference herein in theirentireties.

Growth of nanowire heterostructures in which the different materials aredistributed at different locations along the long axis of the nanowireis described in, e.g., Gudiksen et al. (2002) “Growth of nanowiresuperlattice structures for nanoscale photonics and electronics” Nature415, 617-620; Bjork et al. (2002) “One-dimensional steeplechase forelectrons realized” Nano Letters 2, 86-90; Wu et al. (2002)“Block-by-block growth of single-crystalline Si/SiGe superlatticenanowires” Nano Letters 2, 83-86; and U.S. patent application 60/370,095(Apr. 2, 2002) to Empedocles entitled “Nanowire heterostructures forencoding information.” The disclosures of each of these publications areincorporated by reference herein in their entireties. Similar approachescan be applied to growth of other heterostructures.

As described herein, and throughout co-assigned published PatentApplication Nos. 2007/0212538 and 2008/0280169, the entire contents ofeach of which are incorporated by reference herein, nanowire structureswith multiple shells can also be fabricated, such as, for example, aconducting inner core wire (which may or may not be doped) (e.g., toimpart the necessary conductivity for electron transport) and one ormore outer-shell layers that provide a suitable surface for bindingpolymer electrolyte. Exemplary nanowires that can be used in thepractice of the present invention also include carbon-comprisingnanowires, such as those disclosed in Published U.S. Patent ApplicationNos. 2007/0212538 and 2008/0280169.

In one embodiment, the present invention provides additives for use in abattery slurry. As used herein, an “additive” refers to a compositionthat is added to a battery slurry, such that a portion (e.g., a weight%) of the original slurry is replaced with the additive composition. Asused herein, a “battery slurry” refers to a mixture of components usedto form an electrode (anode or cathode) of a battery.

In an embodiment, the additives comprise one or more carbon-comprising,Si-based nanostructures. As used herein, “carbon-comprising” is used toindicate that the nanostructures comprise carbon in at least some form.Suitably, the nanostructures comprise a carbon shell or sheetsurrounding, or at least partially surrounding the nanostructure. Asused herein, “Si-based” is used to indicate that the nanostructurecomprises at least 50% silicon (Si). Suitably, the nanostructurescomprise at least 55%, at least 60%, at least 65%, at least 70%, atleast 75%, at least 80%, at least 85%, at least 90%, or at least 95% Si.FIG. 1A shows an exemplary carbon-comprising, Si-based nanostructure 100(e.g., a nanowire) of the present invention which comprises a Si core102 and a C shell 104. As show in FIG. 1A, suitably the nanostructuresare Si-based nanowires, however in further embodiments, thenanostructures can be Si-based nanoparticles. It should be noted thatthe carbon-comprising, Si-based nanostructures, as well as othernanostructures described herein, are generically termed “nanostructures”herein.

In another embodiment, the additives comprise one or more nanostructurescomprising a nanoscale scaffold, a Si-based layer disposed on thenanoscale scaffold and a carbon-based layer disposed on the Si-basedlayer. As used herein, a “nanoscale scaffold” refers to a nanostructureupon which one or more other materials, components, layers, coatings,shells, and/or films may be disposed. Example nanoscale scaffoldsinclude nanowires, nanopowder, nanorods, nanofilms, nanotubes, branchednanocrystals, nanotetrapods, tripods, bipods, nanocrystals, nanodots,quantum dots, nanoparticles, branched tetrapods (e.g., inorganicdendrimers), and the like. FIG. 1D shows an exemplary nanostructure 150(e.g., a coated nanowire) of the present invention that comprises ananoscale scaffold 152, a Si-based layer 154 disposed on nanoscalescaffold 152, and a C shell 156 disposed on Si-based layer 154.Suitably, nanoscale scaffold 152 comprises a nanowire (e.g., a Sinanowire), a nanofiber, a nanotube (e.g., a C nanotube), or some othernanoscale scaffold upon which a Si-based layer may be disposed.

The nanostructures of the present invention comprising a nanoscalescaffold, a Si-based layer disposed on the nanoscale scaffold, and acarbon-based layer disposed on the Si-based layer are also referred toherein as nanoscale-scaffold-based compositions, nanoscaffold-basedcompositions, or simply scaffold-based nanostructures.

In exemplary embodiments, the Si-based nanostructures are Si-basednanowires. Exemplary dimensions for the nanowires of the presentinvention are described throughout. Suitably, the nanowires have adiameter of about 10 nm to about 500 nm, or about 20 nm to about 400 nm,about 20 nm to about 300 nm, about 20 nm to about 200 nm, about 20 nm toabout 100 nm, or about 40 nm to about 100 nm. Suitably, the nanowireshave a length of about 100 nm to about 100 μm, or about 1 μm to about 75μm, about 1 μm to about 50 μm, or about 1 μm to about 20 μm. Suitably,the aspect ratios of the nanowires are up to about 2000:1, or suitably,up to about 1000:1, having a diameter of about 20 nm to about 200 nm,and a length of about 0.1 μm to about 50 μm.

Methods for producing nanowires using vapor-liquid-solid (VLS) processesare disclosed, for example, in published U.S. Patent Application No.US-2003-0089899 (the disclosure of which is incorporated by referenceherein). Additional methods for producing nanowires are describedherein, and are well known in the art. In exemplary embodiments, inorder to produce high volume, high density nanowires, methods disclosedin U.S. Provisional Patent Application No. 61/221,501, filed Jun. 29,2009, entitled “Methods for Growth of High Density Nanowires,” AttorneyDocket No. 2132.0680000, the disclosure of which is incorporated byreference herein in its entirety, are used. Following the nanowiregrowth, the nanowires are suitably harvested (e.g., via sonication orother mechanical means). The addition of a carbon-comprising layer(e.g., a C shell) can be added to the nanowires immediately followinggrowth, or after harvesting. The nanowires can then be utilized asadditives as described herein. Additional processing, such as ballmilling, grinding or other mechanical mechanisms to break the nanowiresand additives into smaller pieces or shorter segments can also be used.

As described herein, suitably the additives of the present invention canbe added to currently existing battery slurries, replacing a portion ofthe slurry, e.g., a portion of the graphite component, with thecarbon-comprising, Si-based nanostructure compositions of the presentinvention. Battery slurries utilized in commercial grade batteriesgenerally comprise a mixture of graphite, carbon and a polymer binder(e.g., polyvinylidene difluoride (PVDF)). The amounts and ratios ofthese components generally varies from battery to battery, but slurriesusually comprise about 50%-80% graphite, about 40%-10% carbon and about10% PVDF (all percentages are weight percentages). In exemplaryembodiments, a portion of the graphite component of the slurry issuitably replaced by the additives of the present invention. Forexample, the additives replace about 1 weight % to about 80 weight % ofthe slurry (replacing an equivalent amount of the graphite). Forexample, the additives replace about 1 weight %, about 2 weight %, about3 weight %, about 4 weight %, about 5 weight %, about 6 weight %, about7 weight %, about 8 weight %, about 9 weight %, about 10 weight %, about11 weight %, about 12 weight %, about 13 weight %, about 14 weight %,about 15 weight %, about 16 weight %, about 17 weight %, about 18 weight%, about 19 weight %, about 20 weight %, about 21 weight %, about 22weight %, about 23 weight %, about 24 weight %, about 25 weight %, about26 weight %, about 27 weight %, about 28 weight %, about 29 weight %,about 30 weight %, about 31 weight %, about 32 weight %, about 33 weight%, about 34 weight %, about 35 weight %, about 36 weight %, about 37weight %, about 38 weight %, about 39 weight %, about 40 weight %, about41 weight %, about 42 weight %, about 43 weight %, about 44 weight %,about 45 weight %, about 46 weight %, about 47 weight %, about 48 weight%, about 49 weight %, about 50 weight %, about 51 weight %, about 52weight %, about 53 weight %, about 54 weight %, about 55 weight %, about56 weight %, about 57 weight %, about 58 weight %, about 59 weight %,about 60 weight %, about 61 weight %, about 62 weight %, about 63 weight%, about 64 weight %, about 65 weight %, about 66 weight %, about 67weight %, about 68 weight %, about 69 weight %, about 70 weight %, about71 weight %, about 72 weight %, about 73 weight %, about 74 weight %,about 75 weight %, about 76 weight %, about 77 weight %, about 78 weight%, about 79 weight %, or about 80 weight % of the slurry.

In exemplary embodiments, the additives of the present invention furthercomprise a conductive polymer (e.g., a carbon-based polymer) disposed onthe nanostructures. Exemplary conductive polymers are described hereinand otherwise known in the art, and include, for example, PVDF,polypyrrole, polythiaphene, polyethylene oxide, polyacrylonitrile, poly(ethylene succinate), polypropylene, poly (β-propiolactone), styrenebutadiene rubber (SBR), carboxymethyl cellulose salt (CMC) andsulfonated fluoropolymers such as NAFION® (commercially available fromDuPont Chemicals, Wilmington), polyimide, poly(acrylic acid) etc.Conductive polymers are suitably uniformly dispersed on the surfaces ofthe nanostructures, for example, along the lengths of nanowires. Theinterface between the nanostructures, suitably nanowires, and theconductive polymers also allows for fast charge/discharge cycles of theelectrodes prepared using such materials. In addition, the conductivepolymer coating on the nanowires also helps to accommodate the volumechange in nanowires associated with alkali metal intercalation.

In further embodiments, the present invention provides battery slurriescomprising one or more of the carbon-comprising, Si-basednanostructures, as described herein. Exemplary characteristics of thenanostructures are described throughout. As discussed herein, suitablythe slurries comprise about 1 weight % to about 80 weight % of thecarbon-comprising, Si-based nanostructures (suitably replacing anequivalent amount of the graphite in the slurries).

As described herein, in exemplary embodiments the slurries furthercomprise a carbon-based material in addition to the nanostructuresdescribed throughout. For example, the slurries suitably comprise carbonor graphite in addition to the nanostructures (as well as a polymerbinder).

In another embodiment, the additives of the present invention suitablycomprise one or more nanostructures disposed on a carbon-based substrate(nanostructure—carbon-based substrate compositions). As shown in FIG.1B, additive 110 suitably comprises nanostructures 114 disposed oncarbon-based substrates 112. In additional embodiments, as shown in FIG.1E, additive 110′ suitably comprises nanostructures 114 disposed oncarbon-based powder 112′. Suitably, carbon-based powder 112′ comprisesparticles of about 5 microns to about 50 microns, about 10 microns to 30microns, about 15 microns to about 25 microns, or about 20 microns. Itshould be noted that the components shown in FIGS. 1A-1E are not toscale and provided only for illustrative purposes. As describedthroughout, exemplary nanostructures that can be utilized in thepractice of the present invention include nanowires, nanoparticles ornanofilms.

As used herein, the term “disposed” refers to any method of placing oneelement next to and/or adjacent (including on top of or attached to)another, and includes, spraying, layering, depositing, painting,dipping, bonding, coating, growing, forming, depositing, etc. Suitably,nanostructures are grown on carbon-based substrates. For example, asdescribed throughout, suitably nanostructures 114 are nanowires thathave been grown directly on carbon-based substrate 112, or grownseparately from carbon-based substrate 112, and then disposed (e.g.,attached or otherwise associated) on carbon-based substrate 112.

For example, a catalyst metal, including metal foils or colloids (e.g.,gold colloids) is first disposed on the carbon-based substrate. Then,the catalyst metal is contacted with one or more precursor gases to grownanowires using a VLS-processes. Methods for producing nanowires usingsuch VLS-processes are disclosed, for example, in published U.S. PatentApplication No. US-2003-0089899 (the disclosure of which is incorporatedby reference herein). Additional methods for producing nanowires aredescribed herein, and are well known in the art. In exemplaryembodiments, in order to produce high volume, high density nanowires,methods disclosed in U.S. Provisional Patent Application No. 61/221,501,filed Jun. 29, 2009, entitled “Methods for Growth of High DensityNanowires,” Attorney Docket No. 2132.0680000, the disclosure of which isincorporated by reference herein in its entirety, are used. In suchembodiments, following the nanowire growth, the nanowires can beharvested alone and then disposed on carbon-based substrates, or inother embodiments, the nanowires and the carbon-based substrates ontowhich they are disposed are harvested together. The nanowires and thenanowire-carbon-based substrates can then be utilized as additives asdescribed herein. Additional processing, such as ball milling, grindingor other mechanical mechanisms to break the nanowires and additives intosmaller pieces or shorter segments can also be used.

In an embodiment, Si-based nanostructures are grown on carbon-basedpowder, e.g., graphite powder, without micropores to create a Si-basednanostructure disposed on the graphite powder as shown in FIG. 1E.Suitably, the Si-based nanostructures may comprise Si-based nanowires,Si-based nanofibers, Si particles, Si-based thin layers, and/or Si-basedfilms. In additional embodiments, other materials capable of Liintercalation can be used to grow nanostructures on carbon-comprisingpowder (e.g., graphite powder).

Embodiments of the present invention achieve improved conductivity bygrowing Si-based nanostructures on graphite powder. In addition, theSi-based nanostructure disposed on graphite powder can be used in abattery-electrode slurry and battery-electrode layers, which leveragesthe high capacity of Si and the high conductivity of the graphitepowder.

Additional embodiments of the present invention achieve improvedconductivity by disposing a carbon coating on Si-based nanowires, whichmay also make it easier to integrate Si-based nanowires in carbon-basedslurries for lithium-ion batteries. Lithium-ion battery slurries includea binder, typically comprised of a carbon-comprising polymer (e.g.,styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC),polyvinylidene fluoride (PVDF), etc.). In embodiments, the binder isused as a carbon source for carbon coating Si nanowires. Carbonizing Sinanowires using the battery slurry binder suitably improves theinteraction between the carbonized Si nanowire and the battery slurry.

The compositions of the present invention comprising nanostructuresdisposed on carbon-based substrates are also referred to herein asnanostructure-carbon-based substrate compositions, nanowire-carbon-basedsubstrate compositions, or simply nanostructure compositions or nanowirecompositions.

As used herein a “carbon-based substrate” refers to a substrate thatcomprises at least about 50% carbon by mass. Suitably, a carbon-basedsubstrate comprises at least about 60% carbon, 70% carbon, 80% carbon,90% carbon, 95% carbon or about 100% carbon by mass, including 100%carbon. Exemplary carbon-based substrates that can be used in thepractice of the present invention include, but are not limited to,carbon powder, such as carbon black, fullerene soot, desulfurized carbonblack, graphite, graphene, graphene powder or graphite foil. As usedthroughout, “carbon black” refers to the material produced by theincomplete combustion of petroleum products. Carbon black is a form ofamorphous carbon that has an extremely high surface area to volumeratio. “Graphene” refers to a single atomic layer of carbon formed as asheet, and can be prepared as graphene powders. See e.g., U.S. Pat. Nos.5,677,082, 6,303,266 and 6,479,030, the disclosures of each of which areincorporated by reference herein in their entireties. “Carbon-basedsubstrates” specifically exclude metallic materials, such as steel,including stainless steel. Carbon-based substrates can be in the form ofsheets, or separate particles, as well as cross-linked structures.

Suitably, the nanostructure-carbon-based substrate compositions, thecarbon-comprising, Si-based nanostructures, and/or the scaffold-basednanostructures form an “ink,” which can easily be manipulated and added,i.e., as additive, to various battery slurries, or utilized in thepreparation of battery electrodes, as described herein.

Nanostructures comprising Si (i.e., Si-based nanostructures), includingSi nanowires, and Si nanoparticles, are suitably used in the practice ofthe present invention due to their ability to absorb the volume changeassociated with charging and discharging cycles of a rechargeablebattery. Silicon has the highest known capacity of all materials, andlithiated silicon (Li₁₅Si₄, the highest lithiated phase achievable forambient temperature lithiation of silicon) has a corresponding capacityof about 3579 m Ah/g (milliamp hours per gram) before lithiation. Inaddition, silicon is abundant, inexpensive and safer to manufacture andutilize than graphite. However, from x-ray data, the calculated densityof Li₁₅Si₄ is 1.179 g/cm³. Thus, when silicon is fully lithiated toLi₁₄Si₄ it undergoes a volume expansion of 280% and has a maximumtheoretical volumetric capacity of 2190 mAh/cm³ after lithiation. Thisvolume expansion renders bulk silicon impractical for use in a battery,as the material disintegrates upon repeated charge and discharge cycles,and thus severely limits battery life.

The nanostructures of the present invention—such as, for example,scaffold-based nanostructures, nanowires, including Si nanowires,disposed on carbon-based substrates, or carbon-comprising, Si-basednanowires—absorb the volume change associated with lithiation duringrepeated charge/discharge cycles. The use of carbon-based structuressuch as graphene, carbon-black and other carbon-based substrates, aid inthe absorbance of the volume change in the slurry, as the structures caneasily bend, flex, and deform. In addition, the overall structure of thenanowires allows for volume change within the slurries withoutgenerating excessive forces due to the strain of the wires duringdeformation. A carbon shell or sheet surrounding Si-based nanowires alsoaids in the absorption of volume changes.

As described herein in embodiments, the nanowires for use in thepractice of the present invention have a core-shell structure. Suitably,the nanowires comprise a crystalline core and non-oxide, amorphousshell. That is, the shell of the nanowire does not contain an oxide. Infurther embodiments, the shell can include an oxide, such as withsilicon nanowires. In further embodiments, the nanowires can be of acore-shell-shell structure (or additional shells). Exemplary core-shell(and core-shell-shell) nanowire compositions are described throughout.Suitably the shell of the nanowires is an alkali metal-alloy formingmaterials, such as a lithium alloy-forming material. Suitably a materialthat allows alkali metal (e.g., Li) intercalation, or has a high Lisolubility (e.g., >0.1%). Suitably, the core or a first shell of thenanowires is a good thermal conductor (e.g., thermal conductivity ofk>0.2 Wm⁻¹K (watts per meter*Kelvin)) at room temperature, and a goodelectrical conductor (e.g., resistance of R<5 ohm).

In exemplary embodiments, the carbon-comprising, Si-based nanowires,nanowire-carbon-based substrate compositions, or scaffold-basednanostructures of the present invention form a porous network in whichthe nanowires intertwine, interweave or overlap. This arrangement takesthe form of a porous structure, wherein the size of pores between thenanowires is suitably mesopores and macropores. As used herein the term“mesopores” refers to pores that are larger than micropores (microporesare defined as less than about 2 nm in diameter), but smaller thanmacropores (macropores are defined as greater than about 50 nm indiameter), and suitably have a pore size in the range of greater thanabout 30 nm to less than about 200 nm in diameter. Suitably, thecompositions of the present invention will be substantially free ofmicropores, that is, less than about 0.1% of the pores will bemicropores (i.e., less than about 2 nm in diameter). The porous naturethese nanowire structures allows for increase mass transport ofelectrolyte through the structures, resulting in rapid diffusion of thealkali metal ions.

In suitable embodiments, the nanowires comprise Si, suitably a Si core,and a shell comprising C. While nanowires comprising a SiC core, or aSiC shell can also be used, suitably the nanowires do not comprise acarbide shell (i.e., SP3 carbon), but instead simply comprise a carbonshell (i.e., SP² carbon) that is covalently bonded to the silicon core.In the case of SiC nanowires, the carburization process is suitablycontrolled so as to generate partially carburized Si nanowires (seePublished U.S. Patent Application No. 2008/0280169) with strongly bondedcarbon nanoparticles. Nanowires of the present invention are able toaccommodate the associated volume change with alkali metal (e.g., Li)intercalation. Other suitable materials for use in the core of thenanowires are described herein and include TiO₂.

The use of a core-shell (or multiple shell) nanowire structure in thecompositions of the present invention provide enhanced cycling(charging/discharging) performance, most likely due to the formation ofpassivating films on the surface of the nanowires. An initial capacityloss can originate from the reduction of the electrolyte on the nanowiresurface, resulting in the formation of a solid electrolyte interface(SEI) on the nanowire surface, or from irreversible alkali metal (e.g.,Li) insertion/intercalation into the nanostructures. Preformation of anartificial SEI layer (through chemical modification) on thenanostructures, and or pre-lithiating of the nanostructures, can be usedto enhance performance. In embodiments, a shell comprises a metal and/ormetal oxide, such as Cu, Tin oxide, Ni, and the like. Si nanowiresurface conductivity can be improved in this manner, and reduce thevolume change of the Cu/SiNWs or Ni/SiNWS so that a thin, dense andstable SEI can be maintained on the surface. Such metal and/or metaloxide shells can also reduce the consumption of Li in the battery. Ashell comprising a metal oxide such as tin oxide, for example, alsopermits Li ions to diffuse through the shell, yet prevent solvents inthe electrolyte from penetrating.

In embodiments, nanowires for use in the present invention can furthercomprise nanoparticles on their surface. For example, the nanoparticlescan be graphite or graphene particles or layers. In embodiments wherethe nanowires are used to prepare anodes, as described herein, suitablythe nanowires can further comprise nanoparticles of Si or TiO2 on theirsurface. In embodiments where the nanowires are used to preparecathodes, the nanowires can comprise nanoparticles of LiNiSiO₄, LiNiSiO,LiFeO₂, etc. The nanoparticles decorating the surface of the nanowiresof the present invention are utilized in a highly efficient manner(acting as intercalating or alloying materials) due to the highcurvature of the nanowire surface (e.g., radius of less than about 100nm) on which they are disposed, thus exposing a large number of thenanoparticles to the external environment.

Li—Si alloy compositions can be passivated in polar aprotic electrolytesolutions and by Li-ion conducting surface films that behave like asolid electrolyte interface. Ionic liquids can also be introduced tomodify Si nanowire surface chemistry. Thus, surface chemistrymodification can be realized by tuning components in the electrolytesolutions.

Exemplary dimensions for the nanowires of the present invention aredescribed throughout. Suitably, the nanowires have a diameter of about10 nm to about 500 nm, or about 20 nm to about 400 nm, about 20 nm toabout 300 nm, about 20 nm to about 200 nm, about 20 nm to about 100 nm,or about 40 nm to about 100 nm. Suitably, the nanowires have a length ofabout 100 nm to about 100 μm, or about 1 μm to about 75 μm, about 0.1 μmto about 50 μm, or about 1 μm to about 20 μm. Suitably, the aspectratios of the nanowires are up to about 2000:1, or suitably, up to about1000:1. Such high aspect ratios allow for electrons that are generatedon the nanowires to be rapidly passed between the nanowires to theconducting electrode. In addition, nanowires with diameters of less thanabout 50 nm, and aspect ratios of greater than about 1000:1, demonstrateincreased flexibility when undergoing the volume change associated withcycling between charged and discharged states, as described herein.

In further embodiments, the nanowires for use in the practice of thepresent invention can be porous nanowires, such as porous Si nanowires.Electrochemical cycling during lithiation and delithiation producespores on the walls of the nanostructures. It has been hypothesized thatthe presence of these pores may increase the ability of thenanostructures to accommodate volume changes, and also to increase thesurface area available for contact with conductive polymers and alkalimetals. Preparation of porous nanostructures, including porousnanowires, can be carried out by electrochemical cycling. In anadditional embodiment, a pore forming material can be incorporated intothe nanostructures and then removed to generate the porousnanostructures. For example, Sn or other secondary components can beintegrated into the nanostructures (e.g., Si nanowires), and thenremoved by chemical (e.g., etching) or physical methods. These porousnanostructures, including porous Si nanowires are then utilized in thecarbon-comprising, Si-based nanostructure compositions, thenanostructure-carbon-based substrate compositions, the scaffold-basednanostructures, and additives of the present invention.

Preparation of the additives of the presently claimed invention,including carbon-comprising, Si-based nanowire, nanowire-carbon-basedsubstrate compositions, and scaffold-based nanostructures suitablyutilize nanowire alignment methods such as those disclosed in PublishedU.S. Patent Application No. 2008/0224123 (the disclosure of which isincorporated by reference herein in its entirety) to generate highdensity nanowire compositions. Exemplary alignment methods include theuse of fluid flow and shear extrusion to align the nanowires, as well ase-field alignment and deposition onto various substrates, includingcarbon-based substrates. Spraying can be utilized to introduce nanowiresand/or conductive polymers to the nanowires. The nanowires can also bebent and compressed in order to form a more dense and interwovencomposition.

As described herein, suitably the additives of the present invention canbe added to currently existing battery slurries, replacing a portion ofthe slurry, e.g., a portion of the graphite component. Battery slurriesutilized in commercial grade batteries generally comprise a mixture ofgraphite, carbon and a polymer binder (e.g., polyvinylidene difluoride(PVDF)). The amounts and ratios of these components generally variesfrom battery to battery, but slurries usually comprise about 50%-80%graphite, about 40%¬10% carbon and about 10% PVDF (all percentages areweight percentages). In exemplary embodiments, a portion of the graphitecomponent of the slurry is suitably replaced by the additives of thepresent invention. For example, the additives replace about 1 weight %to about 80 weight % of the slurry (replacing an equivalent amount ofthe graphite).

In exemplary embodiments, the additives of the present invention furthercomprise a conductive polymer disposed on the nanostructures. Exemplaryconductive polymers are described herein and otherwise known in the art,and include, for example, PVDF, polypyrrole, polythiaphene, polyethyleneoxide, polyacrylonitrile, poly(ethylene succinate), polypropylene, poly(βpropiolactone), styrene butadiene rubber (SBR), carboxymethylcellulose salt (CMC), and sulfonated fluoropolymers such as NAFION®(commercially available from DuPont Chemicals, Wilmington), etc.Conductive polymers are suitably uniformly dispersed on the surfaces ofthe nanostructures, for example, along the lengths of nanowires. Theinterface between the nanostructures, suitably nanowires, and theconductive polymers also allows for fast charge/discharge cycles of theelectrodes prepared using such materials. In addition, the conductivepolymer coating on the nanowires also helps to accommodate the volumechange in nanowires associated with alkali metal intercalation.

In further embodiments, the present invention provides battery slurriescomprising one or more of the carbon-comprising, Si-basednanostructures, the nanostructure-carbon-based-substrate compositions,and/or the scaffold-based nanostructures as described herein. Exemplarycharacteristics of the nanostructures are described throughout. Asdiscussed herein, suitably the slurries comprise about 1 weight % toabout 80 weight % of the nanostructure-carbon-based-substratecompositions (suitably replacing an equivalent amount of the graphite inthe slurries).

As described herein, in exemplary embodiments the slurries furthercomprise a carbon-based material in addition to the nanostructuresdescribed throughout. For example, the slurries suitably comprise carbonor graphite in addition to the nanostructures (as well as a polymerbinder).

The present invention also provides battery electrodes comprising one ormore additives of the presently claimed invention (i.e., thecarbon-comprising, Si-based nanostructures, thenanostructure-carbon-substrate based compositions, and/or scaffold-basednanostructures). As shown in FIGS. 1C and 1F, suitably such batteryelectrodes 120 and 120′ are prepared from: the carbon-comprising,Si-based nanostructures 100 of FIG. 1A; thenanostructure-carbon-substrate based compositions 100 shown in FIG. 1Bor 100′ shown in FIG. 1E; and/or the scaffold-based nanostructures shownin FIG. 1D. As shown in FIG. 1F, suitably battery electrodes 120′ mayalso be prepared from nanostructures 114 disposed on carbon-based powder112′. The additives of the present invention can be used to prepareanodes and/or cathodes of batteries. In exemplary embodiments, theelectrodes comprise one type of additive (e.g., only carbon-comprising,Si-based nanostructures 100 of FIG. 1A, thenanostructure-carbon-substrate based compositions 110 shown in FIG. 1Bor 100′ shown in FIG. 1E, or scaffold-based nanostructures 150 shown inFIG. 1D), or in other embodiments, comprise a mixture ofcarbon-comprising, Si-based nanostructures 100 of FIG. 1A, thenanostructure-carbon-substrate based compositions 110 shown in FIG. 1Bor 100′ of FIG. 1E, and/or the scaffold-based nanostructures 150 shownin FIG. 1D.

As described throughout, suitably the nanostructures for use in thebattery electrodes are nanowires, nanoparticles, or nanofilms. Exemplarycompositions of the nanostructures, including core-shell andcore-shell-shell nanowires are described throughout. In exemplaryembodiments, the nanostructures are Si nanostructures, including Sinanowires, and core-shell nanowires in which the core is Si and theshell C, covalently linked to the core. Exemplary sizes of nanowires foruse in the battery electrodes are described throughout.

As described herein, suitably the nanostructure compositions replaceabout 1 weight % to about 80 weight % of the battery slurry. Thus, whenused to prepare a battery electrode, the nanostructure compositions alsocomprise about 1 weight % to about 80 weight % of the electrode,suitably about 5 weight % to about 20 weight %, about 5 weight % toabout 15 weight %, about 5 weight % to about 10 weight %, or about 10weight % of the battery electrode.

In suitable embodiments, compositions of the present invention suitablycomprise a conductive polymer disposed on the nanostructures. Exemplaryconductive polymers are described herein, and include for example,polyvinylidene difluoride, polypyrrole, polythiaphene, polyethyleneoxide, polyacrylonitrile, poly(ethylene succinate), polypropylene, poly(β-propiolactone), styrene butadiene rubber (SBR), carboxymethylcellulose salt (CMC), and sulfonated fluoropolymers such as NAFION®(commercially available from DuPont Chemicals, Wilmington), etc. Theconductive polymer also serves as a binder material when formingelectrodes 120.

In further embodiments, the carbon-comprising, Si-based nanostructures,the nanostructure-carbon-based substrates, and scaffold-basednanostructures of the battery electrodes further comprise an alkalimetal (e.g., lithium (Li), sodium (Na), potassium (K), rubidium (Rb),caesium (Cs), or francium (Fr)). For example, nanostructures of thepresent invention can be embedded in an alkali metal foil, such as alithium (Li) foil. Composites of the nanostructure compositions andalkali metal (e.g., alkali metal films) are highly conductive and asdescribed throughout, demonstrate the ability of the nanostructures(e.g., Si nanostructures) to accommodate the large volume change due toion insertion.

The present invention also provides batteries comprising the variouscarbon-comprising, Si-based nanostructures, nanostructure-carbon-basedsubstrate compositions, and/or scaffold-based nanostructures of thepresent invention. The schematic shown in FIG. 2 of a battery 200 of thepresent invention is provided for illustrative purposes only. Theschematic is not shown to scale, and the orientation and arrangement ofthe battery components are provided only to aid in explanation of thepresent invention. Additional components, orientations and arrangementsof batteries are well known in the art.

In embodiments, as shown in FIG. 2, battery 200 suitably comprises ananode 202. Suitably, anode 202 comprises one or more of thenanostructure compositions, as described herein. In exemplaryembodiments, the anodes comprise the carbon-comprising, Si-basednanostructures. In other embodiments, the anodes comprise thenanostructure-carbon-based substrate compositions. In furtherembodiments, the anodes comprise scaffold-based nanostructures. In stillfurther embodiments, the anodes can comprise any of these differentnanostructure compositions, or multiple anodes, each comprising one orany of these nanostructure compositions. Exemplary nanostructures,including nanowires and compositions of such nanostructures (andnanowires) are described herein. Suitable sizes for nanowires for use inthe batteries are described throughout. In addition, exemplarycarbon-based substrates are also described herein, and include, acarbon-based powder, carbon black, graphite, graphene, graphene powderand graphite foil.

The carbon-comprising, Si-based nanostructure compositions, thenanostructure-carbon-based substrate compositions, and thescaffold-based nanostructures, and additives of the present inventioncan be utilized in any battery type. In exemplary embodiments, thebatteries of the present invention are Li-ion batteries. That is, thebatteries are suitably rechargeable batteries in which a lithium ion 206moves between the anode 202 and cathode 204. The lithium ion moves fromthe anode to the cathode during discharge and in reverse, from thecathode to the anode, when charging.

As described herein, the ability of alkali metals, e.g., Li, to insertinto the nanostructures of the present invention provides increasedcapacity. However, due to the ability of the nanostructures, includingnanowires (e.g., Si nanowires) to absorb the volume change thataccompanies this insertion, the anodes remain structurally sound. Theability of lithium to insert into the nanowires, particularly Sinanowires, provides for a dramatic increase in the capacity of anodesprepared from such materials.

Suitably, the anodes 202 of the batteries 200 of the present inventioncomprise about 1 weight % to about 80 weight % (suitably about 5 weight% to about 20 weight %, or about 10 weight %) of the nanostructures ofthe present invention. A conductive polymer—such as polyvinylidenedifluoride, styrene butadiene rubber (SBR), or carboxymethylcellulose—is also suitably disposed on the nanostructures. In exemplaryembodiments, anode 202 comprises Si nanostructures embedded in a Lifoil.

As shown in FIG. 2, suitably battery 200 further comprises a cathode 204and a separator 208 (e.g., an electrolyte separator) positioned betweenthe anode 202 and the cathode 204 to partition the anode and cathodefrom each other, but also to allow passage of ions through the separator208. In exemplary embodiments, cathode 204 can comprise any suitablematerial known for use as battery cathodes, such as, but not limited to,lithium-based cathodes, such as LiCoO₂, LiFePO₄, LiMnO₂, LiMnO₄,LiNiCoAlO/LiNiCoMnO⁺LiMn₂O₄, LiCoFePO₄ and LiNiO₂. Exemplary materialsfor separator 208 include microporous polymer materials that have goodionic conductivity and sufficiently low electronic conductivity.Suitable materials include PVDF, polypyrrole, polythiaphene,polyethylene oxide, polyacrylonitrile, poly (ethylene succinate),polypropylene, poly (β-propiolactone), and sulfonated fluoropolymerssuch as NAFION®, as well as others known in the art. Battery 200 alsofurther comprises an electrolyte 218, suitably an alkali metal salt(e.g., Li salt) dissolved in an organic solvent, for example, LiPF6 in1:1 w:w, ethylene carbonate:diethyle carbonate. Alternativelyelectrolyte 218 can comprise an alkali metal salt (e.g., Li salt) mixedwith an ionically conducting material such as a polymer or inorganicmaterial so as to form a suspension. Alternatively electrolyte 218 cancomprise additives one or more additives such as ethylene carbonate(EC), propylene carbonate (PC), derivatives or analogous compounds of ECor PC, chloro ethylene carbonate (CEC), dichloroethylene carbonate,fluoro ethylene carbonate (FEC), trifluoro propylene carbonate, vinylenecarbonate (VC), catechol carbonate, ethylene sulfite, propylene sulfite,sulfur additives, diethyl carbonate (DEC), dimethyl carbonate (DMC),derivatives or analogous compounds of DEC or DMC, dimethyl sulfite,diethyl sulfite, S,S-dialkyl dithiocarbonates, methoxyethyl (methyl)carbonate, dimethyl pyrocarbonate (DMPC), dibutyl pyrocarbonate, ethylpropyl carbonate, ethyl methyl carbonate, asymmetric alkyl methylcarbonates, trifluoroethylmethyl carbonate, partially fluorinated linearcarbonates, Li₂CO₃, partially halogenated organic compounds, bromobutyrolactone, chloro ethylene carbonate, fluoro ethylene carbonate,trifluoro propylene carbonate, fluorinated noncyclic compounds, ethers,glycol ethers, urethanes, glycol esters, N,N-dimethylamino trifluoroacetamide, trifluoro ethylmethyl carbonate, partially fluorinated linearcarbonates, methyl chloroformate, fluorinated solvents, vinylenecompounds, acrylic acid nitrile, ethyl cinnamate, vinylene acetate,chlorinated solvents, sulfites, hydrogen fluoride (HF), AlI₃, MgI₂,SnI₂, S_(x) ², 2-methyl-furan (2MeF), 2-methyltetrahydrofuran (2Me-THF),pyridine derivatives, dipyridyl derivatives, cetyltrimethylammoniumchrloride, nonionic surfactants, crown ethers, benzene, CO₂, N₂0, CO,2-methylthiophene (2MeTp), and mixtures thereof.

As shown in FIG. 2, in exemplary embodiments, battery 200 furthercomprises a housing 210 encasing the anode, electrolytic separator andcathode. Suitable shapes and materials for housing 210 (e.g., metals,polymers, ceramics, composites, etc.) are well known in the art andinclude a laminate housing composed of a metallic layer and a syntheticresin layer. For example, a nylon film, an aluminum foil and apolyolefin film layered in this order. The polyolefin film is suitablyfused or bonded by an adhesive to constitute the inner side of thehousing. The polyolefin film may be, for example, a polypropylene film,polyethylene film, or modified polyethylene film. Battery 200 alsosuitably further comprises electrodes 214 and 216, which can comprisemetals such as aluminum, copper, nickel or stainless steel, and connectto load 212.

In embodiments, the present invention provides methods of preparing anadditive for use in a battery slurry. As shown in flowchart 2000 of FIG.20, with reference to FIG. 1E, suitably such methods comprise, in a step2002, providing a carbon-based powder. The carbon-based powder maycomprise particles of graphite, for example, about 5 microns to about 50microns, about 10 microns to about 30 microns, about 15 microns to about25 microns, or about 20 microns in size. In a step 2004, a Si-basednanostructure is disposed on the carbon-based powder. Suitably, theSi-based nanostructure is a Si nanowire or Si nanofiber grown on thecarbon-based powder. Methods of growing Si nanowires are providedherein. Such methods may also optionally include disposing acarbon-comprising polymer on the Si-based nanostructure, as illustratedin a step 2006, and heating the carbon-comprising polymer to form acarbon coating on the Si-based nanostructure. Exemplary heatingtemperatures and times are described herein.

In further embodiments, the present invention provides methods ofpreparing a battery electrode. As shown in flowchart 300 of FIG. 3A,with reference to FIGS. 1A and 1C, suitably such methods comprise, instep 302, providing one or more carbon-comprising, Si-basednanostructures 100. In step 304 of flowchart 300, the nanostructures aremixed with a conductive polymer and a carbon-based material to form aslurry. In step 306, the slurry is formed into battery electrode 120.

Exemplary nanostructures, including nanowires, are disclosed herein, asare compositions and characteristics of the nanostructures. Suitably,the nanostructures are Si nanowires, including core-shell (orcore-shell-shell) nanowires in which the core of the nanowires comprisesSi.

As noted throughout, suitably the carbon-comprising, Si-basednanostructure compositions of the present invention are utilized asadditives in conventional battery slurries to generate electrodes (e.g.,anodes). As noted throughout, suitably such additives are provided atabout 1 weight % to about 80 weight % of the electrode, more suitablyabout 5 weight % to about 20 weight %, or about 10 weight % of theelectrode. As noted herein, suitably the electrodes prepared accordingto the methods of the present invention are anodes of lithium-ionbatteries.

Step 304 of flowchart 300 suitably comprises mixing thecarbon-comprising, Si-based nanostructure compositions with a conductivepolymer such as polyvinylidene difluoride, styrene butadiene rubber(SBR), and/or carboxymethyl cellulose salt (CMC). Other suitableconductive polymers are described herein or otherwise known in the art.The carbon-comprising, Si-based nanostructure compositions are alsosuitably mixed with an additional carbon-based material. Examples ofsuch additional carbon-based substrates are described throughout, andinclude, carbon, carbon black, graphite, graphene, graphene powder orgraphite foil. This combination forms a battery slurry typically used toform electrodes.

The present invention provides further methods of preparing a batteryelectrode.

As shown in flowchart 310 of FIG. 3B, with reference to FIGS. 1B and 1C,suitably such methods comprise, in step 312, providing one or morenanostructures 114 or 114′ disposed on a carbon-based substrate 112 or112′. In step 304 of flowchart 300, the nanostructures are mixed with aconductive polymer and a carbon-based material to form a slurry. In step316, the slurry is formed into battery electrode 120 or 120′.

Exemplary nanostructures, including nanowires, are disclosed herein, asare compositions and characteristics of the nanostructures. Suitably,the nanostructures are Si nanowires, including core-shell (orcore-shell-shell) nanowires in which the core of the nanowires comprisesSi. Exemplary carbon-based substrates are also described herein, andsuitably include carbon black, graphite, graphene, carbon-based powder,graphene powder or graphite foil.

As noted throughout, suitably the nanostructure-carbon-based substratecompositions of the present invention are utilized as additives inconventional battery slurries to generate electrodes (e.g., anodes). Asnoted throughout, suitably such additives are provided at about 1 weight% to about 80 weight % of the electrode, more suitably about 5 weight %to about 20 weight %, or about 10 weight % of the electrode. As notedherein, suitably the electrodes prepared according to the methods of thepresent invention are anodes of lithium-ion batteries.

Step 314 of flowchart 310 suitably comprises mixing thenanostructure-carbon-based substrate compositions with a conductivepolymer such as polyvinylidene difluoride. Other suitable conductivepolymers are described herein or otherwise known in the art. Thenanostructure-carbon-based substrate compositions are also suitablymixed with an additional carbon-based material (in addition to thecarbon-based substrates on which the nanostructures are disposed).Examples of such additional carbon-based substrates are describedthroughout, and include, carbon, carbon black, graphite, graphene,graphene powder or graphite foil. This combination forms a batteryslurry typically used to form electrodes.

As shown in flowchart 400 of FIG. 4A, with reference to FIGS. 1A, 1C and2, the present invention also further provides methods of preparing abattery 200. Suitably, in step 402 of flowchart 400, the methodscomprise providing one or more carbon-comprising, Si-basednanostructures 100. hi step 404, the nanostructures are mixed with aconductive polymer and a carbon-based material to form a slurry. In step406 of flowchart 400, the slurry is formed into an anode 202. Aseparator material 208 is then disposed between the anode 202 and acathode 204 in step 408.

As shown in flowchart 410 of FIG. 4B, with reference to FIGS. 1B, 1C and2, the present invention further provides additional methods ofpreparing a battery 200. Suitably, in step 412 of flowchart 410, themethods comprise providing one or more nanostructures 114 disposed on acarbon-based substrate 112 or 112′. In step 414, the nanostructures aremixed with a conductive polymer and a carbon-based material to form aslurry. In step 416 of flowchart 410, the slurry is formed into an anode202. A separator material 208 is then disposed between the anode 202 anda cathode 204 in step 418.

Methods of preparing lithium batteries, as well as suitable componentsfor cathodes, separator materials, and electrolytes, can be found, forexample, in “Lithium batteries: Science and Technology,” G Nazri and G.Pistoia, eds., Springer, New York (2004), the disclosure of which isincorporated by reference herein in its entirety. These well knownmethods are suitably combined with the methods and nanostructurecompositions described herein to prepare batteries.

As described throughout, exemplary nanostructures include nanowires,nanoparticles, or nanofilms, suitably Si nanostructures, such as Sinanowires, and core-shell (including core-shell-shell) nanowires.Suitable characteristics and sizes of the nanostructures, such asnanowires, are described throughout as well. Exemplary carbon-basedsubstrates are also described herein.

Suitably, the carbon-comprising, Si-based nanostructures, thenanostructure-carbon-based substrate compositions, and thescaffold-based nanostructures of the present invention comprise about 1weight % to about 80 weight % of the anode, suitably about 10 weight %.In suitable embodiments, the nanostructures are mixed with apolyvinylidene difluoride conductive polymer and graphite and/or carbonto prepare the slurry in step 404/414 that ultimately will become thebattery anode. Additional conductive polymers and carbon-based materialsare described herein.

In suitable embodiments, a conductive polymer membrane (separator 208)is disposed between the anode and the cathode in step 408/418. Exemplaryconductive polymer membranes are disclosed herein or otherwise known inthe art.

Methods of preparing the anode, separator and cathode to form thebattery include rolling, heating, drying and storage methods (includingtemperatures and times) that are well known in the art. See for example,“Lithium batteries: Science and Technology,” and U.S. Pat. Nos.6,165,642, 7,541,114 6,440,606, 5,681,357, 5,688,293 and 5,834,135 thedisclosures of each of which are incorporated by reference herein intheir entireties.

As described herein, the carbon-comprising, Si-based nanostructurecompositions, the nanostructure-carbon-based substrate compositions, andthe scaffold-based nanostructures of the present invention are suitablyused as additives in conventional battery electrode (e.g., anode)formation techniques. Thus, these additives are easily substituted inthe battery manufacturing process by simply replacing a portion of thebattery electrode slurry with the presently disclosed additives (e.g.,about 1 weight % to about 80 weight %, suitably about 5 weight % toabout 20 weight %, or about 10 weight %). The remainder of thetraditional battery formation process is then followed. No additionalmodifications are required when using the additives of the presentinvention, though the processes can be further modified if desired.

The present invention provides further methods of preparing acarbon-coated nanostructure. As shown in flowchart 1900 of FIG. 19, withreference to FIG. 1D, suitably such methods comprise, in step 1902,providing a nanoscale scaffold 152. Example nanoscale scaffolds includenanowires, nanopowder, nanorods, nanofilms, nanotubes, branchednanocrystals, nanotetrapods, tripods, bipods, nanocrystals, nanodots,quantum dots, nanoparticles, branched tetrapods (e.g., inorganicdendrimers), and the like.

In a step 1904 of flowchart 1900, a carbon-comprising polymer isdisposed on the nanoscale scaffold 152. The carbon-comprising polymermay comprise, for example, styrene butadiene rubber (SBR), carboxymethylcellulose (CMC), polyvinylidene fluoride (PVDF), and the like, andcombinations thereof.

In a step 1906, the carbon-comprising polymer is heated to form a carboncoating 156 on the nanoscale scaffold 152. The carbon-comprising polymermay be heated to about 160° C. to about 1000° C. For example, thecarbon-comprising polymer may be heated to about 200° C. to about 400°C., about 250° C. to about 350° C., or about 300° C. As another example,it may be heated to about 600° C. to about 800° C., about 650° C. toabout 750° C., or about 700° C. As a further example, it may be heatedto about 800° C. to about 1000° C., about 850° C. to about 950° C., orabout 900° C. The carbon-comprising polymer may be heated for a durationof about 30 minutes to about 5 hours, about 1 hour to about 3 hours, orabout 2 hours. The heating may be done in the presence of an inert gas,such as neon, argon, krypton, or xenon.

As illustrated in a step 1908, such methods may also optionally includedisposing a Si-based layer 154 (e.g., crystalline Si and/or amorphousSi) on the nanoscale scaffold 152, prior to disposing thecarbon-comprising polymer, in which case the carbon-comprising polymeris disposed on the Si-based layer 154.

As described herein, in the Examples set forth below, and in ProvisionalPatent Application No. 61/179,663, filed May 19, 2009, the disclosure ofwhich is incorporated by reference herein in its entirety, the presentinvention overcomes the limitations of bulk silicon and harnesses thehigh charge capacity of silicon in the form of nanowires. The nanowiressuitably have diameters in the 10's of nanometer range, and aspectratios of about 1000. The nanowires are able to absorb the large volumechanges caused by lithiation and de-lithiation during charging anddischarging without losing their structural integrity.

The following Examples describe silicon nanowire (Si NW) performance inpractice. For implementation in Li-ion batteries, the Si NWs aresuitably used as a low-volume (about 5-30 weight %) filler in anodes.This approach allows for easy integration into existing product lineswhile still providing a significant boost in performance. An addition of10% by weight of SiNW has been found to result in a battery anodecapacity boost of about 30%-50%.

It will be readily apparent to one of ordinary skill in the relevantarts that other suitable modifications and adaptations to the methodsand applications described herein can be made without departing from thescope of the invention or any embodiment thereof. Having now describedthe present invention in detail, the same will be more clearlyunderstood by reference to the following examples, which are includedherewith for purposes of illustration only and are not intended to belimiting of the invention.

EXAMPLES Example 1: Preparation and Characterization of Si NanowiresGrowth and Shape of Si Nanowires

Vapor-liquid-solid (VLS) methods are utilized for growing Si nanowires.The wires are single crystalline with diameters between about 20 nm andabout 200 nm and lengths between about 1 pm and about 50 pm. The growthprocesses allow for high degrees of freedom in the design of shape,size, composition etc. For example, nanowires are able to bemanufactured that are substantially straight and with a yield of greaterthan about 99% (see FIG. 5A). For battery applications, an interwoven,interleaving or overlapping structure is suitably used (see FIG. 5B).The nanowires can also easily be doped, as well as grown as alloys ormulti phase materials. Suitably, Si nanowires of approximately 20 nm-60nm diameter in a core/shell configuration where the shell consists of athin layer of carbon that is mostly covalently bonded to the silicon areproduced. This carbon layer provides the path for electronicconductivity.

Suitable Growth Substrates for Silicon Nanowires for BatteryApplications: Carbon Black, Graphite, Graphene

The methods of the present invention can be utilized to prepare siliconnanowires on a variety of different substrate materials and formfactors. For use as an additive to battery slurries, nanowires aresuitably grown onto carbon black, graphite or on loose graphenenanosheet powder surfaces. In all three cases, it is verystraightforward to mix the substrate/Si nanowire compositions/additivesinto anode materials. As described throughout, growing Si nanowires ongraphene or carbon powders allows the structures to accommodate thelarge volume change during lithiation and delithiation of Si materials.Thus, the Si nanowire materials can be utilized directly in graphitebased inks or slurries.

Carbon black is an effective growth substrate for the nanowires as wellas a suitable matrix material for a battery anode, and can easily beintegrated into slurry formulations. The nanowires can be grown oncarbon black, as well as on other substrates, in densities that can beadjusted in a wide range and thus tailored to the desired performancecharacteristics. FIG. 6 shows an SEM micrograph of silicon nanowiresgrown on carbon black. The nanowires form an interwoven and overlappingnetwork that provides a large amount of surface area and accessibilityfor lithiation and ionic and electron transport.

The micrographs in FIGS. 7A and 7B show Si nanowires grown on graphitefoil, at high (A) and low (B) magnification. FIGS. 24A and 24B show Sinanowires at low magnification (A) and high magnification (B). Theaverage diameter of the nanowires is about 45 nm.

FIGS. 8A and 8B show SEM micrographs of loose graphene microsheetpowders (A) and silicon nanowires grown on the graphene powder (B). Theaverage diameter of the nanowires is 50 nm. Si nanowires grown on thenano or micro graphene nanosheet powders provide high surface area andhigh mechanical flexibility to the additives. Both graphite foil andgraphene powder allow for accommodation of the volume change of the Sinanowires and provide high electronic conductivity.

FIG. 9 shows a transmission electron microscopy (TEM) micrograph ofsilicon nanowires 900 with a crystalline core 902 and amorphous shell904. Si nanowires suitably have a core-shell structure with adjustablecore to shell diameter ratios. The core is crystalline and the shellamorphous. The final surface layer is an electronically conductive thinlayer of carbon that is mostly covalently bonded to the shell. Nanowireshave radial dimensions on the order of about one hundred atomic radii,and thus upon lithiation, allow lattice strain to be absorbedelastically. When the strain becomes too large to be accommodatedelastically, a phase transformation from crystalline to amorphous Sioccurs. Upon continuous incorporation of lithium atoms into the crystal,the nanowires eventually accommodate the increasing strain throughplastic deformation and the creation of protrusions or leaf-likestructures 906 extending from the surface of the nanowires.

These protrusions in thin film material result in a reduction of theconductivity of the Si material itself, and hence decrease the capacityof Si material over cycling. However, in the case of Si core nanowirescoated with a C shell, these protrusions provide the benefit of creatingmore surface area and even shorter diffusion paths for the lithium ionsas compared to a smooth nanowire. In addition, loss in electronicconductivity is avoided by the presence of carbon on the wire surfaceand carbon powders or graphite powders in the electrode. This extrasurface area provides an increase, rather than a decrease, in thecapacity of a battery anode containing Si nanowires of the presentinvention with increasing number of charge cycles.

Nanowires provide a continuous electronic conduction path that does notappear to be achieved at the same level with spherical nanoparticles.Spheres by their very nature only have a limited number ofsphere-to-sphere point contacts that contribute to electronicconductivity. Nanowires also provide an additional degree of freedom inadjusting the porosity.’

FIGS. 25A and 25B show TEM micrographs of silicon nanowires with acrystalline core (about 15 to 20 nm thick) and a combination of anamorphous (Si—O may be involved) and poly-Si shell (about 10 to 15 nmthick) covered by a carbon shell. Referring to FIG. 25A, siliconnanowire 2500 has a crystalline core 2502 of approximately 14.76 nm witha carbon shell, comprising a first side 2504 and a second side 2506. Thefirst side 2504 of the carbon shell has a thickness of approximately13.2 nm, and the second side 2506 of the carbon shell has a thickness ofapproximately 10.03 nm. Referring to FIG. 25B, silicon nanowire 2550 hasa crystalline core 2552 of approximately 19.44 nm with a carbon shell,comprising a first side 2554 and a second side 2556. The first side 2554of the carbon shell has a thickness of approximately 13.93 nm, and thesecond side 2556 of the carbon shell has a thickness of approximately11.42 nm.

Capacity of Si Nanowires on a Stainless Steel Electrode with LithiumFoil Counter Electrode

In order to measure the charge capacity and cycle efficiency of Sinanowires, the nanowires were grown on a steel substrate as an anode,and used in conjunction with lithium foil as the counter electrode.

FIG. 10 shows the charge capacity and cycle efficiencies for nanowireswith two different diameters. The thinner (40 nm) diameter wires (soliddiamonds in the figure) achieve the theoretical capacity of bulk siliconof 4200 mAh/g maximum capacity during the first cycle, with a firstcycle efficiency of 85% (open squares in the figure). The chargecapacity decreases with increasing number of cycles, which is anartifact of the experimental arrangement and caused by the lack ofbinder and additives. Thicker (80 nm) nanowires demonstrate a smaller(2110 mAh/g) initial capacity (solid circles in the figure) thatincreases with increasing charge cycle number. This behavior can beunderstood by the fact that the diffusion distances for Li are longerand the strain relaxation via surface protrusions more difficult. Thefirst-cycle loss in this case is 15% as well (open circles in thefigure).

These measurements clearly demonstrate the theoretical charge capacitycan be demonstrated with 40 nm thick wires. As described below, these Sinanowires have been used to develop an electrode that provides anenhanced capacity that can be maintained for 80 cycles without little tono reduction.

Comparison of Silicon Nanowires with Silicon Thin Films and Powder

Si nanowires behave quite differently than Si thin films, bulk Si, or Sipowders. When silicon nanowires are produced on a stainless steelsubstrate, a thin layer of silicon is also produced on the stainlesssubstrate between the bases of the nanowires. The measurements providedherein therefore contain contributions from both the nanowires and theSi thin film. FIG. 11 shows the current versus potential curves taken at0.1 mV/s for Si NWs with different diameters. The sharp peaks at 0.48 Vare directly related to the Si nanowires. The feature at 0.29 V is thesignature of silicon in the form of thin films. For very thin wires, thevolume fraction of the thin film becomes large enough for itscontribution to contribute to the current versus potential scan. Thevery large current at the charging peak of the silicon is part of thereason why nanowires allow for quick charging.

A distinctly different behavior can be seen in FIG. 12, where a siliconthin film without nanowires is compared to a thin film with nanowires.For the scan on the sample with nanowires, the extra peak close to 0.5 Vis observed.

FIG. 26 shows Fourier Transform Infrared Spectroscopy (FTIR)measurements, illustrating differences between SiNWs and Si powders. TheFTIR suggests that Si—O stretches at approximately 1070 cm⁻¹, indicatingthat SiO₂ may exist in the shell materials.

Carbon Coating on Si Nanowires

A carbon coating suitably improves the conductivity of Si nanowires andthe ability to integrate the Si nanowires into carbon-based slurries forlithium-ion batteries. A carbon-based polymer (such as, SBR, CMC, PVDF,etc.) is typically used as a binder in the battery slurries. Inembodiments, the binder is used as a carbon source for carbon coating onthe Si nanowires. Carbonizing the carbon-based polymer to form a carboncoating on Si nanowires may also improve the interaction between thecarbon-based polymer and the carbon coating on the Si nanowires.

FIGS. 21A and 21B are micrographs that show Si nanowires 2104 that havea carbon coating 2102. The carbon coating 2102 was achieved bydecomposing SBR (styrene butadiene rubber). In this example, Sinanowires 2104 were mixed with SBR and then heated to about 700° C. for2 hours in the presence of Argon to form carbon coating 2102.

FIG. 22 is a micrograph that shows Si nanowires 2204 that have a carboncoating 2202. The carbon coating 2202 was achieved by decomposing PVDF(poly(vinylidenefluoride)). In this example, Si nanowires 2204 weremixed with PVDF and then heated to about 900° C. for 2 hours in thepresence of Argon to form carbon coating 2202.

Si nanowires that have a carbon coating (e.g., a surface layer of carbonor a carbon shell) show a better cycling performance when used in alithium-ion battery. The improved performance may be due to formation ofpassivating films on the surface of electrode materials, especially forSi and carbon-coated Si.

FIGS. 23A and 23B are micrographs that show Si nanowires of embodimentsof the present invention after several charge/discharges cycles. FIG.23A illustrates that significant morphology changes did not occur after2 charge/discharge cycles. FIG. 23B illustrates that the Si nanowiresbecame more porous after 10 charge/discharge cycles.

FIGS. 23C and 23D are micrographs that show carbon-coated Si nanowiresof embodiments of the present invention after several charge/dischargecycles. The Si nanowires in these figures were carbon coated usingdecomposed PVDF. After 2 charge/discharge cycles, as illustrated in FIG.23C, there was not significant morphology changes in the carbon-coatedSi nanowires, and the carbon coating is still intact. After 10charge/discharge cycles, as illustrated in FIG. 23D, there was notsignificant morphology changes in the carbon-coated Si nanowires, butthe carbon coating split along the length of the Si nanowires.

Example 2: Preparation and Characterization of Anodes UsingNanowire-Carbon-Based Substrate Additives Increased Anode Capacity andCycle Life

To approximate commercial battery formulations and for purposes of abaseline and control, a mixture of 80% graphite with 10% carbon and 10%PVDF (Li-G-C-PVDF) was utilized as a battery slurry. To determine thecapacity of the nanowire materials of the present invention, 10% of thegraphite was replaced with 10% Si Nanowire material (Li-SiNW-G-C-PVDF).FIG. 13 shows a resulting increase in capacity when using the nanowires.The capacity gain is initially 30% and continues to increase to 50%after about 60 charge/discharge cycles. The increase in capacity withnumber of cycles can be explained by examining an SEM micrograph of thestructure of the Si NWs after a few cycles (FIG. 14). The formerlysmooth nanowire surfaces become microstructured, increasing the surfacearea, thus increasing the interfacial area between silicon and ionconductor and shortening the diffusion paths for Li in the Si nanowires.

FIG. 27 shows a graph of capacity as a function of cycle number for afirst anode comprising 10% Si nanowires, 10% PVDF, and 80% graphitecarbon, and a second anode comprising only graphite carbon and PVDF. Thecycling performances were obtained with the cells after 10 constantvoltage (CV) and 3 constant current (CC) cycles. The cycling resultsshown in FIG. 27 were tested using CC cycles of about 1.5 hours/halfcycle. For the first anode (comprising Si nanowires, graphite, andPVDF), a capacity gain of more than 30% was achieved in 250 cycles.

Li Ion Battery: Fast Response Rates to Current Pulse for NanowireMaterial

FIG. 15 shows the charge cycling behavior of a Li SiNW anode/Li CoO2cathode cell as compared to the same cell without nanowires as control.The cell containing the nanowires in the anode exhibits very fastresponse rate to various current pulses (e.g., at 1 mA in 3-second timeslots). This fast rate can be attributed to a large surface area andshort diffusion paths for Li ions, as well as a unique network structureproviding efficient electronic conduction.

Uniform Distribution of Polymer Binder

As described herein, suitably the nanowires of the additives of thepresent invention are arranged in an interwoven, interleaving oroverlapping network. For an efficient battery design, however, it isimportant to uniformly distribute carbon as well as the binder (e.g.,conductive polymer or “polymer binder”). In order to demonstrate theuniform distribution of polymer, lead (Pb)-stained NAFION® was used as amodel substance that could be tracked using Energy Dispersive X-Ray(EDX) analysis. FIGS. 16A-16C show scanning transmission electronmicroscope (STEM) EDX elemental maps of Si nanowires (16A), carbon(16B), and Pb (16C) demonstrating the uniform distribution of C andbinder on the Si nanowires. The nanowire network surface area can beadjusted to 30-100 m²/g for 20-60 nm nanowires, which is substantiallylarger than that of graphite powders (˜1 m²/g) in commercial batteries.

Heat-treated battery electrodes may improve binder distribution and,therefore, may lead to better cycling. In one example, a foil withPVDF-SiNW-graphite-conductive carbon black was heated at 300° C. under4% hydrogen in Argon for 8 hours. The melting point of PVDF is about160° C. The onset temperature of PVDF decomposition is higher than 350°C., so 300° C. is an effective temperature for heat treatments ofembodiments of the present invention.

Heat-treated Si nanowire-graphite-PVDF electrodes may improve adhesionto the current collector (e.g., Cu) and, more importantly, may make arelatively dense/uniform coating layer. The improved adhesion of thecoating layers on the current collector may lead to better cyclingperformance. In addition, a better interaction between the binder andactive material powders may also result in reduced changes in the solidelectrolyte interphase (SEI), which also influences cyclingperformances.

Manufacturing and Integration into Existing Slurry Preparations

As described herein, Si nanowires can be disposed on a number ofsubstrates. Using the methods described throughout, nanowire diameter(e.g., 20-200 nm), length (e.g., to about 50 um), taper (usuallytargeted to be zero), size distribution (>+/−20% full width athalf-maximum), and doping (if desired), over a wide range and with highyield, can be readily controlled. Nanowires with Si cores and SiC shellswith adjustable core/shell ratios and graphitic surface layers can bereadily produced. Production output has been scaled-up by 100× from thelab scale and successfully tested in a prototype for a manufacturingline designed for high volume (50 tons of Si NWs per year) production.

An exemplary manufacturing process is shown in FIG. 17. This processutilizes a high volume, high density method of growing nanowires asdisclosed in U.S. Provisional Patent Application No. 61/221,501, filedJun. 29, 2009, entitled “Methods for Growth of High Density Nanowires,”Attorney Docket No. 2132.0680000, and in U.S. Provisional PatentApplication No. 61/179,663, filed May 19, 2009. The growth of Sinanowires suitably utilizes nanowire nucleation from gold colloidcatalysts in a silicon-rich chemical vapor deposition environment. Asset forth in FIG. 17, the production methods shown in flowchart 1700,suitably comprise step 1702, in which an aluminum foil is embossed. Instep 1704, the foil is then cleaned using conventional solvents, and instep 1706 a substrate surface is prepared (e.g., a carbon-basedsubstrate). Gold colloid is disposed on the substrate in step 1708,followed by drying in step 1710. Nanowire growth is then performed instep 1712 using a VLS-process (other processes as described herein canalso be used). The nanowires are then harvested (for example, bysonication) in step 1714, filtered in step 1716 and dried in step 1718.The nanowires can then be balled milled in step 1720 to be used asadditives as described herein. Suitably, as described in U.S.Provisional Patent Application No. 61/221,501, filed Jun. 29, 2009,entitled “Methods for Growth of High Density Nanowires,” Attorney DocketNo. 2132.0680000, and U.S. Provisional Patent Application No.61/179,663, a cartridge assembly 1722 is used to facilitate preparationof a large number of nanowires.

FIG. 18 describes an exemplary process of introducing the additives ofthe present invention into existing slurry preparationprotocols/equipment designs 1800. As shown in FIG. 18, exemplarypreparation protocols/equipment design 1800 suitably includes pump 1802,powder transfer blowers 1804 and 1810, and positive slurry mixer 1806and negative slurry mixer 1808. The positive and negative slurry mixersfeed into slurry pumps 1812 and 1814, respectively. Slurry pumps 1812and 1814 feed into positive coater dryer 1818 and negative coater dryer1820, respectively. A solvent recovery mechanism 1816 is also provided.Positive coater dryer 1818 and negative coater dryer 1820 both feed intoroll storage 1822, which ends the exemplary preparationprotocol/equipment design 1800. As described throughout, additives ofthe present invention are suitably added in 1824 to powder transferflower 1810, which are then mixed and prepared into anodes. Othersuitable preparation protocols/equipment designs will be readilyenvisioned by those skilled in the art, and the design shown in 1800 isprovided for illustrative purposes only as an example.

Exemplary embodiments of the present invention have been presented. Theinvention is not limited to these examples. These examples are presentedherein for purposes of illustration, and not limitation. Alternatives(including equivalents, extensions, variations, deviations, etc., ofthose described herein) will be apparent to persons skilled in therelevant art(s) based on the teachings contained herein. Suchalternatives fall within the scope and spirit of the invention.

All publications, patents and patent applications mentioned in thisspecification are indicative of the level of skill of those skilled inthe art to which this invention pertains, and are herein incorporated byreference to the same extent as if each individual publication, patentor patent application was specifically and individually indicated to beincorporated by reference.

1. A battery anode electrode comprising: an additive comprising one ormore Si-based nanostructures grown directly on the surface of acarbon-based substrate and being attached thereto, said carbon basedsubstrate comprising graphite particles having a diameter of 5 micronsto 50 microns, and wherein the Si-based nanostructures have a core-shellstructure and comprise Si-based nanowires having a diameter of 20 nm to100 nm and a length of 0.1 μm to 50 μm; a polymer binder; and acarbon-based material comprising at least one of carbon black, graphite,graphene, graphene powder and graphene foil; wherein the additivefurther comprises a conductive polymer disposed on the nanostructuresand wherein the weight % ratio of the additive in the electrode isbetween 1% and 80%.
 2. The battery anode electrode of claim 1, whereinthe weight % ratio of the additive in the electrode is between 5% and20%.
 3. The battery anode electrode of claim 1, wherein the core of thenanostructures comprise silicon and the shell of the nanostructurescomprise carbon.
 4. The battery anode electrode of claim 1, wherein thecore of the nanostructures comprise crystalline silicon and the shell ofthe nanostructures comprise amorphous silicon.
 5. The battery anodeelectrode of claim 1, wherein the Si-based nanowires form a porousnetwork wherein the size of the pores is at least one of mesopores andmacropores.
 6. The battery anode electrode of claim 1, wherein theSi-based nanostructures are embedded in an alkali metal foil. Thebattery anode electrode of claim 6, wherein the alkali metal foil is alithium foil.
 8. The battery anode electrode of claim 1, wherein theconductive polymer comprises at least one of PVDF, polypyrrole,polythiophene, polyethylene oxide, polyacrylonitrile, poly(ethylenesuccinate), polypropylene, poly (β-propiolactone), styrene butadienerubber (SBR), carboxymethyl cellulose salt (CMC) and sulfonatedfluoropolymers.
 9. The battery anode electrode of claim 1, wherein theconductive polymer is a coating disposed on the nanostructures.
 10. Thebattery anode electrode of claim 1, wherein the conductive polymer isuniformly dispersed on the surfaces of the nanostructures.
 11. ALithium-ion battery comprising the anode electrode according to claim10, a cathode, and an electrolyte.
 12. The lithium-ion battery of claim11, wherein the electrolyte comprises one or more selected from thegroup consisting of (a) an alkali metal salt dissolved in organicsolvent, (b) an alkali metal salt mixed with an ionically conductingpolymer, and (c) an alkali metal salt mixed with an ionically conductinginorganic material.
 13. The battery anode electrode of claim 1, whereinthe Si-based nanostructures comprise a combination of the Si-basednanowires and Si-based nanoparticles having all dimensions less than 50nm.
 14. A method of making a Lithium-ion battery anode electrodecomprising: providing an additive comprising one or more Si-basednanostructures grown directly on the surface of a carbon-based substrateand being attached thereto, said carbon based substrate comprisinggraphite particles having diameter of 5 microns to 50 microns, andwherein the Si-based nanostructures have a core-shell structure andcomprise Si-based nanowires having a diameter of 20 nm to 100 nm and alength of 0.1 μm to 50 μm. disposing a conductive polymer on theSi-based nanostructures; heating the conductive polymer in a presence ofan inert gas at a temperature between 160° C. and 1000° C. for aduration of about 30 minutes to about 5 hours; mixing the additive witha polymer binder and a carbon-based material comprising at least one ofcarbon black, graphite, graphene, graphene powder and graphene foil; andforming the slurry into a battery anode electrode, wherein the weight %ratio of the additive in the slurry is between 1% and 80%.
 15. Themethod of claim 14, wherein the heating comprises heating the conductivepolymer in a presence of an inert gas at a temperature between 200° C.and 400° C. for a duration of about 1 hour to about 3 hours.
 16. Themethod of claim 14, wherein the conductive polymer comprise any of PVDF,polypyrrole, polythiophene, polyethylene oxide, polyacrylonitrile,poly(ethylene succinate), polypropylene, poly (β-propiolactone), styrenebutadiene rubber (SBR), carboxymethyl cellulose salt (CMC) andsulfonated fluoropolymers, or combination thereof.
 17. The method ofclaim 14, wherein the heating forms a coating on the Si-basednanostructures.
 18. The method of claim 14, wherein the wherein theSi-based nanostructures comprise a combination of the Si-based nanowiresand Si-based nanoparticles having all dimensions less than 50 nm.